As discussed in the co-pending patent application corresponding to Provisional Patent Application Ser. No. 61/299,652, filed Jan. 29, 2010 as signed to the assignee hereof and incorporated herein by reference, a nuclear quadrupole resonance detection system is described in which signals having energy corresponding to expected spectral lines for a target molecule are transmitted down a balanced terminated transmission line. This has been found to produce stimulated emissions for any target molecules within the transmission line, with the stimulated emissions detected utilizing a directional coupler in one embodiment.
As described in the above patent application, it is only with difficulty that one can detect molecules when for instance injecting energy in the 0.1-5 MHz range to cause the stimulated emission. It has been found however that such a stimulated emission does occur and is recognizable such that utilizing the above basic NQR techniques and a network analyzer one can detect the signature of the stimulated emission response in terms of the S21 response of the analyzer.
In order to be able to detect the stimulated emission response and more particularly to be able to distinguish it from background clutter, in one embodiment of the basic NQR system one monitors the balanced transmission line without any material therein, thus to provide a reference level for the transmission line. One then operates the system to detect any target molecules within the confines of the transmission line by comparing the S21 ratio from the network analyzer, with the self-same S21 ratio obtained when none of the target molecules exist within the transmission line.
While such stimulated emissions have been found, it is important to be able to distinguish the stimulated emission peaks or spectral lines in the presence of clutter and more particularly to be able to do a comparison between the empty transmission line and one in which target molecules are present.
This comparison is not necessarily straight forward due to the fact for instance that the molecules when they are packaged or carried on a person have an associated stimulated emission response that contains a dielectric component. This dielectric component alters the shape of the stimulated response curve such that the reference when subtracted from the detected signal does not yield exact cancellation of environmental clutter. This permits ready isolation of the spectral line.
Aside from the dielectric component surrounding the molecules in question, there is also the problem of finding a peak or spectral line that is some 70 to 80 dB down and embedded in noise. The noise level in this case refers to clutter components and in such systems the clutter noise level may be as high as at −10 dB.
On the other hand, the spike or spectral line that is sought to be detected may be buried in this noise and may be some 80 dB down, meaning that the detectable signal is more than 70 dB below the noise level.
The question then becomes how to robustly identify spectral lines that are buried so far beneath the clutter.